Where does the question come from?
The evolution of the Tethys, ancient oceans existing for at least the past 500 Myr, transferred continents from the south to the north hemisphere. The most important fact about the Tethyan Ocean is that it is an equatorial ocean with single-directed (northward) subduction (Wu et al., 2020). The Tethyan subduction products are not similar to those of the Andean margin, for example, there are rarely porphyry copper deposits (PCD) formed during the Tethyan subduction stage. PCD is generally believed to be formed in an oxidized environment, which is a common product in the Andes subduction system. Instead, the Tethyan region is extremely enriched with Sn deposits, which are generally related to reduced magma. Therefore, does the equatorial ocean subduction prefer to make reduced magma? Actually, the latitude effect is important for the Earth’s surface environment, which determines the net primary production in the ocean (organic material), a key reductant for the solid Earth. Therefore, the Tethyan solid Earth evolution and surface latitude environment changes should have strong interactions (Wan et al., 2023). Taking into account the above issues, we decided to look at the global data (circum-pacific belt) to see whether there is any relationship between the latitude and redox state of arc magma.
The global data compilation.
Taking advantage of the global database, such as GEOROC, we can easily obtain enormous data of magmatic rocks from all over the world. Before the compilation, we took a look at some previous studies working on the oxygen fugacity of arc magma (Stopler and Bucholz, 2019). It showed that the oxygen fugacity of Cenozoic magma was quite varied, which gives us confidence to dig into the potential reasons for this phenomenon. We used two well-established whole-rock geochemical parameters, V/Sc and Cu/Zr, to reflect the oxygen fugacity of the arc magma. The data are selected to represent the primary magma composition, which is important for comparing their oxygen fugacity at different locations, after filtering the data by widely agreed-upon criteria. Additionally, limited olivine-hosted melt inclusion data is also collected. The results showed that both whole-rock and melt inclusion data indicate a latitude-dependent oxygen fugacity of arc magma with less oxidized magma in lower latitudes (Fig. 1). We were excited about this observation because it fits perfectly with our original ideas, and the pattern is even better. As a result, the latitude effect of solar radiation could be reflected in the deep Earth’s mantle.
Fig. 1. Relationships between the ΔFMQ of olivine-hosted melt inclusions (a) and V/Sc of basaltic rocks (b) and latitude.
The potential mechanism.
We originally thought that the mechanism should be straightforward, with more organic carbon deposited and subducted in the lower latitude regions, causing the sub-arc mantle to be less oxidized. However, the available data from the ocean showed that there is a clear discrepancy between the sediments deposited on the ocean floor and trenches. Indeed, more organic carbon is deposited on the lower latitude ocean floor, but such a pattern is not observed in the trench sediments yet (Fig. 2, Clift, 2017). Then, we find out that microbial sulfate reduction is quite common in marine sediments, and more organic sediments in the lower latitudes may result in more reduced sulfur in the lower latitudes (Kasten and Jørgensen, 2000). We then look at the data on the sulfur isotope of the arc magma. The sulfur isotope of melt inclusions does show that more reduced sulfur is input in the lower latitudes (Fig. 2). Thus, we suggested that more sulfate is reduced by organic carbon, resulting in less oxidized sub-arc mantle and arc magma in the lower latitudes (Fig .3).
Fig. 2. The flux of particulate organic carbon (POC) in the ocean (a), carbon in subducted sediments (b), and variation in average δ34S (‰) of arc magma (c) across latitude.
Fig. 3. The schematic cartoon diagram of the influence of solar radiation on deep Earth (Credit: Jiancong Liang).
The future work.
Although the proposed mechanism could explain our observed pattern, the available data is still relatively limited. Therefore, we think continuous accumulation of compositional data of arc magma, marine sediments, and subducted sediments is necessary and important. Moreover, our findings may provide critical insights into mineral deposits (e.g., Cu, Sn, and Li), which may help locate the deposits formed million years ago. Another thing is that it shows us that environmental changes on the Earth’s surface may have a great impact on the deep Earth. Last, we are still trying to find other potential influences caused by the latitude effect of solar radiation on the deep Earth.
References:
Wu, F. Y., Wan, B., Zhao, L., Xiao, W. J., & Zhu, R. X., (2020). Tethyan geodynamics. Acta Petrologica Sinica, 36(6), 1627-1674.Wan, B., Wu, F. Y. & Zhu, R. X., (2023). The influence of Tethyan evolution on changes of the Earth’s past environment. Science China Earth Sciences, 53(12), 2653-2665.Stolper, D. A., & Bucholz, C. E. (2019). Neoproterozoic to early Phanerozoic rise in island arc redox state due to deep ocean oxygenation and increased marine sulfate levels. Proceedings of the National Academy of Sciences, 116(18), 8746-8755.Clift, P. D. (2017). A revised budget for Cenozoic sedimentary carbon subduction. Reviews of Geophysics, 55(1), 97-125.Kasten, S., & Jørgensen, B. B. (2000). Sulfate reduction in marine sediments. In Marine geochemistry (pp. 263-281). Berlin, Heidelberg: Springer Berlin Heidelberg.